1. Field of the Invention
This invention relates to microspheres composed of a plurality of inorganic colloidal particles and to a process for their manufacture which proceeds by coacervation of organic material into microspheres containing the colloidal inorganic particles. The composite structures containing the organic material and the colloidal inorganic particles are useful for a variety of purposes including flame resistant and pigmenting fillers. Porous microspheres, formed by burning out the organic material from the composite structures are useful for a variety of purposes including catalytic and chromatographic packings such as those disclosed in U.S. Pat. No. 3,782,075 for Completely Porous Microspheres for Chromatographic Uses filed on the same day as the present application by J. J. Kirkland, the disclosure of which is hereby incorporated by reference into the present specification.
2. Description of the prior art
Powders composed of porous silica microspheres having a diameter in the range of 0.1 to 50 microns have been made by spray drying sols containing colloidal silica. As disclosed in U.S. Pat. No. 3,301,635, for Molded Amorphous Silica Bodies and Molding Powders for Manufacture of Same, which issued to H. E. Bergna et al on Jan. 31, 1967, the microspheres produced by this process are nonuniform aggregates of colloidal silica particles which combine to form a powder having a tapped bulk density of at least 0.2 gms/cm..sup.3. The microparticles are lightly coalesced structures having large nonuniform pores. Upon compaction at elevated temperature they form very dense silica bodies with smaller yet more uniform pores.
Another process for forming extremely small porous silica particles is disclosed in U.S. Pat. No. 3,591,518 for Silica Aquasols and Powders which issued to D. McMillan on July 6, 1971. This process comprises providing a heel containing silica sol and aqueous ammonium hydroxide, introducing finely-divided silicon metal into the heel and permitting the metal and water to react in the presence of ammonia to form silica. The silica so formed polymerizes on the heel rather than form new nuclei so the size of the particle can be controlled. The product formed is a powder of small porous particles, having a diameter of 0.15 to 0.50 microns, each of which is made up of an aggregate of still smaller ultimate particles having a diameter of 2.5 to 6.0 millimicrons.
Still another process for the formation of porous inorganic oxides is disclosed in U.S. Pat. No. 3,417,028 for a Process for the Preparation of an Inorganic Gel Having a Predetermined Pore Structure which issued to D. S. Montgomery on Dec. 17, 1968. This process proceeds by adding a water soluble organic polymer to a hydrous gel of the oxide, dehydrating the mixture until it sets, then removing the organic material by thermal decomposition. The structure formed is in the form of a filter cake with a controlled pore size distribution and an enlarged pore volume.
Finally, U.S. Pat. No. 2,383,653 for Silica Acid Sols which issued to J. S. Kirk on Aug. 28, 1945 discloses a process for producing microscopic spheres by precipitation from a N,N'-bis(methoxymethyl) uron, polysilicic acid solution N,N'-bis(methoxymethyl) uron is a derivative of urea and formaldehyde and polysilicic acid contains silica particles of molecular size. According to Kirk, the product formed is an interpolymer of polysilicic acid and N,N'-bis(methoxymethyl) uron. He does not discloses thermal decomposition of the organic material, but as shown below, the pore diameter of such a product would be extremely small and the particles would sinter to a nonporous state at elevated temperatures.
The silica microparticles formed by the processes of the prior art are generally either very dense structures or they are highly porous structures of nonuniform diameter. The present invention deals with a new process for the formation of oxide microparticles, having a diameter in the 0.5 to 20 micron range, which have a uniform size. In its preferred aspects, it relates to the formation of oxide powders comprising porous, uniform-sized microspheres having a controlled uniform pore size and a large specific surface area.
More specifically, microspheres in the 0.5 to 20, preferably 1 to 10, micron range made by the process of the present invention from colloidal particles in the 5 to 500 millimicron size range have a unique combination of uniform sphere diameter and pore dimensions. The pore diameter is large relative to the surface area within the sphere which allows rapid diffusion of materials to all parts of the inner surface